Field of the Invention
The invention relates to mixed sugar fermentation, in particular the fermentation of a sugar composition comprising glucose, galactose and arabinose. The sugar composition may originate from ligno-cellulosic material.
Description of Related Art
Most of the ethanol produced as alternative for fossil fuels is currently from fermentation of corn starch and sugar cane based sucrose. In order to reach the ambitious goals for producing renewable fuels, new technologies are being developed for converting non-food biomass into fermentation products such as ethanol. Saccharomyces cerevisiae is the organism of choice in the ethanol industry, but it cannot utilize five-carbon sugars contained in the hemicellulose component of biomass feedstocks. Hemicellulose can make up to 20-30% of biomass, with xylose and arabinose being the most abundant C5 sugars. Heterologous expression of a xylose isomerase (XI) is an option for enabling yeast cells to metabolize and ferment xylose. Likewise, expression of bacterial genes araA, araB, and araD in S. cerevisiae strains results in utilization and efficient alcoholic fermentation of arabinose. Galactose is a C6-sugar that is also a sugar that is often present in lignocellulose, often in amounts (˜4% of total sugars) that are not to be neglected for economic reasons.
J. van den Brink et al, Microbiology (2009)155, 1340-1350 discloses that glucose is the favoured carbon source for Saccharomyces cerevisiae and that upon switching from glucose limited fermentation conditions to galactose-excess condition under anaerobic condition, galactose was not consumed.
Sofar no process has been disclosed to convert galactose, into the fermentation product in the same process with glucose and one or more C5 sugar. An object of the invention is therefore to provide a process to convert galactose into the fermentation product in the same process with glucose and one or more C5 sugar.